Wireless networks are used in many fields of application, the nodes thereof communicating via a wireless radio interface. For example, wireless sensor networks or sensor-actuator networks are used for monitoring and controlling production plants, chemical processes or for monitoring pipelines. These sensor nodes and/or sensor-actuator nodes communicate wirelessly with one another via a radio interface in order to exchange measured values and control commands.
Such wireless networks generally comprise network nodes, the energy supply thereof being provided by a battery contained in the node. In such battery-operated network nodes, the lifetime and/or operating time thereof is restricted by the energy stored in the battery. As soon as the battery is empty, the circuits contained in the network node are not able to be supplied with power and no longer function. In order to increase the lifetime of a network node, therefore, the network node is under certain circumstances transferred to a sleep operating mode in which components and/or circuits within the network node are at least partially deactivated so that the energy consumption of the network node is reduced. In this connection, one of the circuit arrangements of the network node for a wireless network is formed by the radio interface and/or a transmission and receiving unit within the network node. As said transmission and receiving unit consumes a relatively large amount of energy, high energy savings and/or a significant increase in the lifetime may be achieved, if the transmission and receiving unit is transferred to a sleep operating mode and/or deactivated. A deactivation of the transmission and receiving unit, however, restricts the accessibility and/or the communication with the respective network node. Further components, such as for example a processing unit (microprocessor), may also be transferred into an energy-saving sleep state.
In order to permit the deactivation of the transmission and receiving unit and/or the radio module of a wireless network node and at the same time to ensure the accessibility and/or a wireless communication with the network node, therefore, a wireless network node is conventionally provided with an additional transmission and receiving unit and/or receiving component which is only provided to wake the respective network node from a sleep state and/or to transfer it from a sleep operating mode into a normal working operating mode. This additional transmission and receiving component is also referred to as the wake-up-radio and/or as the wake-up radio interface. In order to communicate with the network node, therefore, a transmitter which, for example, is located in a different network node, transmits a wake-up signal to the wake-up transmission and receiving component of the network node to be woken up, the received network node to be woken up being transferred and/or switched by the wake-up signal from the sleep operating mode into a normal working operating mode. As soon as the network node is in the normal working operating mode, the transmitter is able to communicate the desired message to the network node, which is received by the activated transmission and receiving device provided therefor and/or the radio module of the network node. The wake-up radio and/or the additional transmission and receiving component, which is provided for the wake-up signal, is designed such that it consumes considerably less energy than the usual main transmission and receiving unit for receiving messages.
This conventional method for activating a network node by means of a wake-up signal, however, provides the opportunity for the network node and/or the wireless network to be attacked by a third party. An attacker may namely exhaust the limited energy reserves of the battery-operated network nodes of the wireless network in a relatively short time, by waking up the network nodes continuously from the sleep operating mode by transmitting a wake-up signal and transferring them to the normal operating mode. Such an attack is also referred to as a sleep deprivation attack.
A method for the secure waking-up of a computer by a network card of the computer is disclosed in U.S. Pat. No. 6,493,824 B1, in which a data packet is received from a network, a destination address contained in the data packet is compared with a destination address of the network card and a wake-up pattern is sought within the data packet. If the destination addresses coincide and a wake-up pattern is found, an encrypted value of the data packet is decrypted and compared with an expected value. If the values coincide, a signal is generated for waking up the computer.